1. Field of the Invention
This invention relates to a control system for a vehicle automatic transmission, and more particularly to a control system for a vehicle automatic transmission which always determines the optimum gear ratio particularly during hill descending.
2. Description of the Related Art
There have been proposed various vehicle automatic transmission control systems. One example of such a system is described in Japanese Patent Publication No. Sho 59-8698. The assignee proposed a similar system in Japanese Laid-Open Patent Publication No. Hei 5-71626, which was also filed in the United States and matured to Patent (U.S. Pat. No. 5,317,937). In the system there are prepared in advance separate gear shift scheduling maps (gear shift diagrams or programs) for different road profiles--one for level-road driving, another for up-hill driving, and so on, and a parameter indicative of driving resistance such as vehicle acceleration is obtained. It is then estimated on the basis of the parameter if the vehicle is hill climbing or hill descending and based on the determination, one map is selected from among them and a gear ratio is determined in accordance with the selected map. More specifically, the vehicle acceleration is predicted from the vehicle operating conditions including engine output torque and is compared with the actual vehicle acceleration to estimate the degree of hill climbing/descent, i.e., the road profile on which the vehicle is traveling, on the basis of which map among all stored maps most suitable for the traveling condition is selected and a gear ratio is determined on the basis of the selected map.
Aside from the above, it is proposed by Japanese Laid-Open Patent No. Sho 53-35866 a vehicle automatic transmission control system. In the system, a gear is forcibly downshifted if a brake is depressed during hill descending in which the vehicle speed increases even after the throttle valve has been closed. In the vehicle automatic transmission control disclosed in Japanese Laid-Open Patent Publication No. Hei 5-71626 proposed by the assignee, if the gear shifting control is conducted in the manner as taught by the above reference (53-35866), a forcible downshifting will often occur no matter how the grade of downhill is. However, since this forcible downshifting is not the one immediately intended by the driver, the driver may experience an unpleasant feeling. This unpleasant feeling should preferably be avoided for the driver. Moreover, downshifting makes the vehicle travel in a lower gear than usual and use of a lower gear causes higher engine speed, which increases engine noise. In particular, the steeper downhill the grade is, the more quickly the engine speed rises and hence the higher the engine speed becomes.